It is well known that a high voltage, in the range of a few thousand volts, applied to a sharp point produces ions in the surrounding air. A lot of negative ion generators based on this principle exist in the marketplace. They generally consist of a high voltage power supply of which its negative terminal is connected to a sharp point, usually that of a needle, and its positive terminal is connected to an electrode, typically a ring (collector), in front of the needle. Sometimes this is duplicated a few times in a typical unit. Many negative ion generators of which I am aware, mainly for car use, are of this type.
Having the positive electrode, a ring, in front of the negative needle, a short path is made from the needle(s) to the ring(s). This causes two major problems. First, the ions coming out of the needle(s) get readily absorbed by the ring(s) (collectors) and much less ions get through the ring(s) to be of benefit, resulting in inefficient ion generation. Second, the proximity of the opposite collector electrode (ring(s)) to the emitter(s) creates a high differential voltage gradient which, due to the high velocity of the ions bombarding the oxygen molecules of the air between the electrodes, produces ozone.